Close a race where one thread registers a kevent and blocks in
f_attach allowing another thread to register a change or deletion
on the same event.

* delete race

Close a race where one thread deletes a kevent and blocks in
f_detach allowing another thread to register a change or deletion
on the same event.

* scan/close race

Close a race where one thread is in the middle of a kqueue_scan()
and another concurrently close()s a related file descriptor. This
could cause select and poll to loop forever due to not being able
to delete the 'spurious' kevent.

* register/close race

Close a race where one thread registers a kevent and blocks
in f_attach and another thread close()s the related file
descriptor at the same time.